1. Field of the Invention
The present invention relates to poly(lactic acid) fibers having satisfactory mechanical properties at high temperatures.
2. Description of the Related Art
Strong demands have been made on polymer materials that are decomposed in the environment and are thereby environmentally friendly. As possible candidates therefor, aliphatic polyesters and other polymers have been investigated, developed and been launched. Among them, polymers that are decomposed by microorganisms, i.e., biodegradable polymers have become a focus of attention.
Most of conventional polymers are made from petroleum resources. However, the petroleum resources are limited and will probably be exhausted in the future. In addition, the petroleum resources are derived from hydrocarbons in fossils in a geologic age and have been accumulated in the ground, and heavy consumption and burning of the petroleum resources invites emission of carbon dioxide into the atmosphere to thereby cause global warming. If polymers can be synthesized from vegetable resources that take in carbon dioxide from the atmosphere for their growth, such vegetable-origin polymers are expected to decrease carbon dioxide in the atmosphere as a result of xe2x80x9ccarbon dioxide circulationxe2x80x9d and to solve problems of the exhaustion of the petroleum resources. Polymers derived from the vegetable resources, i.e., biomass-derived polymers have therefore received attention.
Such biomass-derived biodegradable polymers receive great attention and are expected to be an alternative to conventional polymers derived from the petroleum resources. However, such biomass-derived biodegradable polymers generally have insufficient mechanical properties and heat resistance and require high cost for their production. The most noteworthy polymer as a biomass-derived biodegradable polymer that can solve these problems is poly(lactic acid). The poly(lactic acid) is a polymer derived from lactic acid, which lactic acid can be obtained by fermenting starch extracted from vegetable. The poly(lactic acid) has the best balance in mechanical properties, heat resistance and cost among such biomass-derived biodegradable polymers. Fibers using the poly(lactic acid) have been developed at a feverish pace.
However, even the most promising poly(lactic acid) has some disadvantages as compared with the conventional polymers. One of serious disadvantages is insufficient mechanical properties at high temperatures. The phrase xe2x80x9cinsufficient mechanical properties at high temperaturesxe2x80x9d used herein means that the poly(lactic acid) rapidly becomes soft at temperatures exceeding 60xc2x0 C., i.e., the glass transition temperature (Tg) of the poly(lactic acid). With reference to FIG. 3, when a conventional poly(lactic acid) fiber is subjected to a tensile test at different temperatures, the poly(lactic acid) fiber rapidly becomes soft at temperatures in the vicinity of 70xc2x0 C. or higher and becomes nearly fluid and exhibits markedly deteriorated dimensional stability at 90xc2x0 C. In contrast, a fiber of nylon 6 (polyamide 6), a conventional polymer, does not become soft so rapidly and exhibits sufficient mechanical properties even at 90xc2x0 C.
The poly(lactic acid) fiber has insufficient mechanical properties such as strength and creep resistance at high temperatures as mentioned above and actually invites problems. For example, when the poly(lactic acid) fiber is used as the warp of woven fabrics, the warp is sized and dried with hot air for better condensing and better weaving. However, upon hot air drying, the warp poly(lactic acid) fiber elongates by action of tension applied to stretch the warp taut. When products made from the poly(lactic acid) fiber are used in a high-temperature atmosphere, they have some problems in their durability. For example, Kogyo Zairyo (Industrial Materials), No. 6, p82 (2001) mentions that the inside temperature of cars in summer reaches 72xc2x0 C. on the surface of a front seat and 80xc2x0 C. on the surface of an upper side of a rear seat. When the poly(lactic acid) fiber is used as a fabric for car seats, the resulting car seats have insufficient durability, since the surface temperatures of car seats exceed Tg of the material poly(lactic acid).
These problems significantly limit the applications of the poly(lactic acid) fiber. Accordingly, demands have been made on poly(lactic acid) fibers having improved mechanical properties at high temperatures.
Japanese Unexamined Patent Application Publication No. 2000-248426 discloses a high-strength yarn obtained by multistage drawing of a poly(lactic acid) undrawn yarn formed by low-velocity spinning. However, the results in further testing made by the present inventors show that even a high-strength yarn having a strength of 7 cN/dtex obtained by multistage drawing does not have practically satisfactory mechanical properties at high temperatures (Comparative Example 1). However, differences in mechanical properties at high temperatures cannot be explained by strength at room temperature alone, since such a high-strength poly(lactic acid) yarn has insufficient mechanical properties at high temperatures, but a high-strength poly(ethylene terephthalate) yarn has satisfactory mechanical properties at high temperatures. Thus, insufficient mechanical properties at high temperatures are unique to the poly(lactic acid) fibers.
Accordingly, it is an object of the present invention to provide a poly(lactic acid) fiber having satisfactory mechanical properties at high temperatures.
Specifically, the present invention provides, in an aspect, a poly(lactic acid) fiber having a strength at 90xc2x0 C. of equal to or more than 0.8 cN/dtex.
The present invention further provides, in another aspect, process for producing a poly(lactic acid) fiber. The process includes the step of drawing a poly(lactic acid) undrawn yarn at such a drawn ratio (DR) as to satisfy the following condition:
0.85+(EL/100)xe2x89xa6DRxe2x89xa62.0+(EL/100)
wherein EL is the elongation (%) of the undrawn yarn.